1. Field of the Invention
The present invention relates to an inverter device comprising a parallel circuit of a charging device and a switching device connected between a DC power source and an inverter, and to an inverter module comprising a switching element group provided in a mold package to convert a DC voltage into a three-phase pseudo AC voltage by switching.
2. Description of the Related Art
Recently, an air conditioner equipped with an electrically driven compressor driven by a battery power source has been developed as an air conditioner for an electric automobile. This air conditioner comprises, as shown in FIG. 7, a battery (DC power source) 401, an inverter device 408 and an electrically driven compressor 410.
The inverter device 408 comprises a switch 402 as a switching device connected in serial to the battery 401; a charging device 411 connected in parallel to this switch 402 and including a resistor 404 and a switch 403; an inverter module 405 configured by molding, into a mold package 415, a switching element group 412 including a switching element 414 and an unshown diode to absorb a switching surge; and a capacitor 406. The switching element group 412 of the inverter module 405 converts a DC voltage from the battery 401 into a three-phase pseudo AC voltage, and applies it to the electrically driven compressor 410 to drive a motor of the electrically driven compressor 410.
The capacitor 406 stably supplies a voltage to the switching element group 412. The resistor 404 of the charging device 411 restricts an incoming current through the capacitor 406 when the DC voltage of the battery 401 is applied. That is, the presence of the resistor 404 makes it possible to, when the battery 401 is connected, open the switch 402, close the switch 403, and pass a current via the resistor 404, thereby restricting the incoming current produced when the voltage of the battery 401 is applied (e.g., refer to Japanese Patent Publication Laid-open No. 3341327).
On the other hand, in such an air conditioner, the switch 402 might be welded to remain in a closed state despite a command being issued by an unshown controller to open the switch 402. Thus, heretofore, voltage dividing resistors have been provided before and after the switch 402, and terminal voltages of these voltage dividing resistors have been detected to check the occurrence of welding of the switch 402. That is, voltage detection circuits are respectively connected between a positive line 416 that is located between the switch 402 and the previous battery 401, and a negative line 418, and between the positive line 416 that is located between the switch 402 and the subsequent switching element group 412, and the negative line 418 of the battery 401, and the voltage detected by each of the voltage detection circuits is input to the controller.
Here, in a state where the battery 401 is connected, the voltage input from the voltage detection circuit before the switch 402 to the controller is substantially the voltage of the battery 401, regardless of whether the switch 402 is opened or closed. The voltage input to the controller from the voltage detection circuit after the switch 402 will be zero if the switch 402 is opened, and the voltage will be substantially the same as the voltage from the voltage detection circuit before the switch 402 (with a slight voltage drop) when the switch 402 is closed. This makes it possible to detect whether the switch 402 is opened or closed from the voltages of the voltage detection circuits before and after the switch 402 input to the controller. It is thus possible to judge the state where the switch 402 is closed even though the switch 402 is controlled to be opened by the controller, the so-called welding of the switch 402.
However, the high-capacity capacitor 406 is placed in the inverter device 408 as described above, and a measured voltage is not immediately reduced due to a discharge voltage of the capacitor 406, so that it has not been possible to detect the welding of the switch 402 in early stages.
On the other hand, in such an air conditioner, when the battery 401 is reversely connected, the capacitor 406 and the switching element group 412 are damaged. Further, there is a disadvantage that when a person touches a terminal on the battery 401 side, the person gets an electric shock because of the charged capacitor 406. In order to prevent this, conventionally, a diode directed forward to the capacitor 406 side has sometimes been attached to the charging device 411.
However, the addition of the diode leads to an increase in size of the entire device. On the other hand, especially in the air conditioner for the electric automobile, because the inverter device is installed on a vehicle to drive the electrically driven compressor, it has to be placed in a small engine room having limited installation space, so that the size increase is not allowed, and a further size reduction has been needed.